The present inventions relate to a highly reflective coating, with a diffuse reflectivity and good resistance to high temperatures, techniques for preparing and applying the coating (e.g. as a paint), light reflectors coated with the material on one or more surfaces, as well as lighting systems or other energy transducer systems incorporating reflectors manufactured with such surface coatings.
Radiant energy transducer systems find a wide range of applications in modern technology. Electrically driven transducers, for example, emit radiation to illuminate a desired area or footprint. The transducer system may illuminate the area for a number of reasons. For example, if the emitting transducer emits visible light, the illumination may facilitate use of the area by human personnel. If the illumination of the area provides infrared radiant energy, the illumination may facilitate some associated detection operation or human monitoring of the area through special night vision equipment.
Many such systems require a diffuse reflectivity on one or more surfaces within each system, and for efficiency, the reflectivity of such surfaces must be relatively high. In at least some applications, the reflective surfaces are subject to high-intensity light and attendant high temperatures.
One example of a material with a high diffuse reflectivity is SPECTRALON, and use of this material has been suggested for optical transducer systems, including light systems. Attention is directed for example to commonly assigned U.S. Pat. No. 6,064,061 to Ramer et al. This material, however, generally has to be molded and machined, and is not easily applied as a coating. Such molding and/or machining is often too expensive for mass production applications, for example, for lighting fixtures.
It has been known to use a variety of white paints for forming necessary reflector surfaces. U.S. Pat. No. 5,967,652 to Ramer et al. suggested that diffusely reflective surfaces for a luminaire could be constructed of a suitable base material of, for example, aluminum or plastic, with a coating of a diffuse reflective material such as barium sulfate or quasi-diffuse reflective materials, such as white paint.
However, readily available coating materials, particularly those available in the quantities and at the prices appropriate for mass production use, have suffered from inadequate reflectivity and/or excessive sensitivity to temperature. In a luminaire, for example, if the diffuse reflectivity of a reflective surface is not high enough, then the lamp within the luminaire must emit more light to achieve a desired illumination performance characteristic. If the material providing the diffuse reflectivity is too sensitive to temperature, it can not withstand the heat from certain types of high intensity lamps. If overheated, the material may break down and become even less reflective (e.g., turn brown), or the material may even catch fire. To avoid these consequences, using existing reflective materials, luminaires often may need to be designed to provide relatively large spacings of the reflectors from the lamps, to allow for heat dissipation. This approach, however, increases the overall size, weight and complexity of the luminaire.
Exotic materials are known, which may be spray-painted under carefully controlled conditions and exhibit both a high-reflectivity and high resistance to temperature. However, such materials have been extremely expensive, as well as difficult and expensive to apply. As a result, such materials have not been utilized in optical transducer systems, like mass produced luminaires.
For example, U.S. Pat. No. 4,111,851 to Shai discloses a coating characterized by low thermal absorption and high thermal emittance. This patented coating comprises: (a) a fired oxide pigment of aluminum oxide and zinc oxide, a vehicle-binder comprising an alkali metal silicate; and sufficient water to provide a mixture suitable for application to a substrate, such as aluminum. The patent, however, teaches use of this material for coating the surfaces of spacecraft. U.S. Pat. Nos. 5,296,285 and 5,885,658 to Bable et al. teach complicated techniques for coating metal substrates with similar white paints having low solar absorbance and high heat emittance. All three patents teach use of these materials and complex application techniques for thermal control applications on the exteriors of spacecraft.
Another example of an available inorganic white paint for such spacecraft applications is the paint marketed as Z-93 by Illinois Institute of Technology Research Institute (IITRI), which apparently comprises calcined zinc oxide particles and a potassium silicate binder. The calcination process involves firing or baking the zinc oxide, in excess of 1000xc2x0 C. for greater than 8 hours, to remove impurities. Because of the time and energy required to produce calcined zinc oxide this step increases the overall cost of the material significantly, and in turn the cost of the coating. The Z-93 material currently sells for $125 per pint. Also, this material requires a very complex and expensive process to mix and apply. For example, IITRI specifies a mixing period of 6 to 8 hours in a ball end mill (a drum filled with the coating and porcelain balls). Although the material is sprayable or spot brushable, IITRI specifies very precise temperature and humidity requirements during application and curing of their paint. For example, the coating must cure at  greater than 50% for the first 5-6 hours and at ambient conditions for seven days. These requirements for preparing, applying and curing the Z-93 material radically increase the complexity and cost of manufacturing components with such a coating. While these costs may be acceptable in one-off applications, such as spacecraft, they make the use of the Z-93 coating in mass-production applications completely impractical, economically.
Hence, there is a continuing need for highly efficient radiant energy transducer systems. To support that general need, there is an attendant need for radiant energy transducer systems and for reflectors for such systems which have desirable diffuse reflective properties yet are easy and cost effective to mass produce. Hence, a need exists in the context of radiant energy transducer systems, for an easily applicable coating material, of relatively low cost, which exhibits diffuse reflectivity, is highly reflective and is relatively stable when exposed to high levels of light and heat. There is an attendant need for techniques to mix and apply such reflective materials to substrates of reflectors, in a manner that is efficient and cost effective in a mass production environment.
The inventive concepts address the stated needs and alleviate the above noted problems with the prior art by providing a paintable highly reflective diffuse coating material, processes for the mixing and application thereof, reflector products coated with the material and radiant energy transducer systems (e.g. luminaires) that incorporate such reflectors.
Several aspects of the present invention also envision application of the inventive reflective materials to surfaces of optical distribution devices and/or optical sensors that require optical surfaces that have a diffuse reflectivity and are highly reflective. The material is particularly advantageous in systems where the dimensions and illumination power create a high temperature, and therefore, the material of the reflective surface must withstand such a high temperature.
For example, one aspect of the present invention relates to a radiant energy transducer system, comprising a radiant energy reflector, having a diffusely reflective area. The system includes a transducer, associated with the reflector, for transducing between radiant energy reflected from at least a portion of the diffusely reflective area and an electrical signal corresponding to the reflected radiant energy. In an emitter type system, such as a visible light luminaire, the transducer comprises one or more lamps. In the context of a sensor or detector system, the transducer comprises one or more energy detectors, such as photodiodes or other photodetectors.
In the inventive system, the reflector comprises a substantially rigid substrate, for example constructed of steel or plastic, or of an aluminum or aluminum alloy, shaped to provide the desired configuration for a particular energy processing application. A surface of the substrate is configured to provide a predetermined shape for the diffusely reflective area. In a luminaire for example, the substrate may form a cavity in the shape of a portion of a sphere or cylinder, or the substrate may form a parabola. The inventive system also includes a diffusely reflective coating on the surface of the substrate, to provide the desired reflective characteristics for the reflector. The coating material includes a zinc-oxide based pigment, consisting essentially of an uncalcined zinc oxide and preferably containing a small amount of a dispersing agent. The pigment is mixed with an alkali metal silicate vehicle-binder, which preferably is a potassium silicate, to form the coating material. When the pigment and binder are mixed with an appropriate amount of water, the liquid form of the material forms a paint for easy application to surfaces to provide the desired reflective and heat resistant properties.
Other inventive aspects relate to a material or paint for application to a substrate of a reflector for a radiant energy transducer system. The coating material exhibits a diffuse reflective characteristic, a high reflectivity to radiant energy and a high stability when exposed to relatively high temperatures.
The ratio of weight of pigment to weight of binder preferably is between approximately 1.41:1 and 1.15:1. Within this ratio range, a most preferred or typical value for this weight ratio is around 1.28:1. These weight ratios apply both in the liquid (paint) state of the material and in the dry coating state on the surface of the reflector.
The preferred embodiment utilizes a pigment that includes a small amount of a dispersing agent, such as propryonic acid. For example, the pigment which is predominantly uncalcined zinc oxide may also include approximately xc2xd% by weight of the propryonic acid.
Sufficient water is included, to provide a mixture suitable for application to the substrate of the reflector. The material preferably forms a water soluble, sprayable paint. The amount of water may be increased to facilitate other forms of application, for example, to allow dipping of articles such as light bulbs in the liquid material to coat the desired surface thereof.
The preferred range of acceptable amounts (%) of water, for dilution of the mixture in the liquid state is between a minimum of 7% and a maximum slightly higher than 40% (40.14% in a specific example). A more preferred range is from 22.63% to 25%, with the typical percentage value for the water around 24%.
In the various embodiments, the reflector, with the coated substrate provides diffuse reflectivity xe2x89xa795%. To provide the desired reflectivity, the coating should be 2.75 mils or greater in thickness. However, if it is desired to provide a semi-reflective semi-transmissive coating, for example on a transparent substrate (e.g. the glass globe of a light bulb), then the coating can be applied to a thickness of  less than 1 mil. In such a case, some light passes through the substrate and the coating, and some light is reflected. Using the preferred zinc oxide-potassium silicate coating compositions, the materials can substantially withstand temperatures up to at least 250xc2x0 C. The preferred formulation of the material has been tested and found to withstand temperatures as high as 700xc2x0 C.
The use of the uncalcined zinc oxide, instead of the calcined zinc oxide, as in the prior art materials (e.g. the IITRI coating) substantially reduces the cost of the material, yet allows the material to provide the performance desired for lighting and other radiant energy system applications.
Other aspects of the invention encompass techniques for mixing and applying the material to form reflectors, so as to facilitate practical, economical mass-production. Such an aspect, for example, might relate to a method of manufacturing a reflector for use in a radiant energy transducer system. The preferred method involves forming a substantially rigid substrate, having a surface configured to provide a predetermined reflector shape. An uncalcined zinc-oxide pigment is mixed with an alkali metal silicate vehicle-binder and water in a shear mixer. The resulting mixture may be painted onto the surface of the substrate to form a diffusely reflective coating, for example by spraying, dipping or brushing.
Because the paint is water-based, it is water soluble and therefore easy to handle and clean-up, in an industrial paint type application. Also, the material is relatively non-toxic. The paint and its processing are relatively in-expensive. Yet the product provides a high degree of reflectivity and a high tolerance to heat.
It is contemplated that the coating may be used in virtually any type of radiant energy transducer system requiring a diffuse reflectivity. The preferred embodiments described in detail below show just a few specific types of such systems, by way of example.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.